gitignore: Ignore default bsimt_bt test output folder

Similarly to sanitycheck the bsim_bt tests have their own
work folder where compilation output and coverage results are
kept.
Ignore the default folder

Signed-off-by: Alberto Escolar Piedras <alpi@oticon.com>
diff --git a/.gitignore b/.gitignore
index 4e8485a..f515658 100644
--- a/.gitignore
+++ b/.gitignore
@@ -30,6 +30,7 @@
 doc/latex
 doc/themes/zephyr-docs-theme
 sanity-out*
+bsim_bt_out
 scripts/grub
 doc/reference/kconfig/*.rst
 doc/doc.warnings